The present invention relates to a focus error detector, and more particularly, a focus error detector which detects a signal representing the degree of a focus error of an objective lens with respect to the recording medium using a critical angle method. The focus error detector may be used in conjunction with an optical pickup for recording and reproducing information by scanning a recording medium using optical spots.
A conventional method for detecting a focus error in an optical recording and reproducing pickup employs a critical angle method using a critical angle prism. In a focus error detector according to the critical angle method, the angle of the reflecting mirror surface of a prism is tilted to be close to a critical angle with respect to an optical axis. A simple example according to this conventional technology is shown in FIG. 1.
The light radiated from a laser diode 1 (a light source) is collimated by means of a collimating lens 2, passed through a beam splitter 3 and then directed to be incident on an objective lens 4. Thereafter, the light is focused onto an optical disk 5 by the objective lens 4. Reflected light from the optical disk 5 is collimated again via the objective lens 4, the reflected light reflects onto the beam splitter 3, and then directed toward a critical angle prism 6. Then, the light further reflects onto the critical surface 6a of the critical angle prism 6 to finally reach a bisectional photo-detector 7. A differential amplifier 8 differentiates signals of divisional sections 7a and 7b of the bisectional photo-detector 7. At this time, the polarity and magnitude of the signals of the differential amplifier 8 vary according to the degree of the focus error of the objective lens, which depend on the distance between the objective lens 4 and optical disk 5, as described in detail with reference to FIGS. 2 and 3.
FIG. 2A shows an in-focus state, i.e., the state when the optical disk is positioned on the focal plane of the objective lens. At this time, the reflected light 9 which is collimated is totally reflected from the critical surface 6a of the critical angle prism 6. Therefore, as shown in FIG. 3A, the reflected light 9 is uniformly received by both divisional sections 7a and 7b of the photo-detector 7, and the relative input signal value of the differential amplifier 8 becomes zero. However, if the optical disk deviates from the focal plane of the objective lens, the reflected light 9 from the optical disk is converged or diverged as shown in FIGS. 2B and 2C. At this time, the reflection amount at point A or B on the critical surface 6a of the critical angle prism 6 is decreased. Therefore, as shown in FIGS. 3B and 3C, the reflected light 9 is received in a semicircular shape by one or the other of divisional sections 7a and 7b of the photo-detector 7, and the relative input signal value of the differential amplifier 8 becomes negative or positive.
Over optical disk vibration and mutation causes the optical axis of the reflected light to shifted laterally or tilt. As a result, the reflection efficiency of the critical prism with respect to the reflected light is changed. Accordingly, although the optical disk is positioned in the focal plane of the objective lens, there is a difference in the amount of light received by the respective divisional sections, which registers a disparity between the input signal values to the differential amplifier. That is to say, the focus error detector according to the conventional critical angle method recognizes the shift or tilt of the optical axis of the reflected light as a focus, which results in the malfunction of the optical pickup.